Research On Slope Length Effect On Runoff And Sediment Yield And Dynamics Processes

The study of slope length effect on runoff, sediment yield and dynamics process is the breakthrough to reveal erosion mechanism on slope and is the basic problem to solve sediment transport mechanics, soil erosion and soil and water conservation. The proven of slope length effect on runoff, sediment yield and hydrodynamics process under different land use types and solution of conversion coefficients of runoff modulus and soil erosion modulus between indoor and field experiments will provide scientific basis for prediction and conservation of soil and water loss in red soil region.Based on the systemic summary of correlative researches at home and abroad, this study mainly analyzed2issues, which included10chapters. The first one discussed the soil erosion intensity and their varieties versus slope length on sloping farmland, economic forest slope and bare slope, which are the primary land use types in red soil region, southern China. The second issue paid more attention to reveal the difference value of soil and water loss between indoor and field experiments and calculate the conversion coefficients between them. For the first issue, we designed5slope lengths and5rainfall intensities to discuss slope length effect on runoff, sediment yield and dynamics processes though artificial rainfall simulation (total of164rainfall events and150were effective) and relationships between hydrodynamics parameters and runoff and sediment yield. Based on above experimental results, the comparison of erosion intensities on three land use types was analyzed. For the second issue, experiments of slope length effect on runoff and sediment yield and dynamics processes on indoor bare slope, which were corresponding to field bare slope(total of30rainfall events and30were effective), were carried out. Compared indoor and field experiment results, the conversion coefficients of runoff modulus and erosion modulus were calculated and thus be used to predict more scientific and exact field soil loss values, which were obtained according to indoor simulated experiment results multiplied by conversion coefficients. And more reasonable soil and water conservation measures could be distributed on slope. And the main results are as follows:1. Soil erosion intensity analysis on three land use typesEconomic forest slope, sloping farmland and bare slope were chosen as the research objects, we studied runoff and sediment yield process on three land use types and compared the soil erosion modulus of individual rainfall event between them. Results showed that the order of erosion modulus for three land use types was sloping farmland>bare slope>economic forest slope under the same rainfall situation. The increment and increase velocity with rainfall intensity were highest on sloping farmland then on bare slope and the economic forest slope was the lowest. The increase velocity order of erosion modulus with every slope length increment2m under30-120mm/h rainfall intensities was sloping farmland>bare slope>economic forest slope, whereas, the order was economic forest slope>sloping farmland>bare slope under150mm/h rainfall intensity. Rainfall intensity had more impact on erosion modulus than slope length.2. Dynamics process analysis of slope runoff and sediment yieldThe runoff velocity was measured as original value to calculate hydrodynamic parameters, which represent runoff dynamic process, to further analyze the flow pattern, relationship between shear stress and slope length, relationship between runoff depth and soil erosion modulus and the impact effect of hydrodynamic parameters on sediment yield on slopes. Results demonstrated that runoff shear stress increased totally with increasing slope length and the relationship between soil erosion modulus and runoff depth could be described by power function(R2>0.85) on three land use types. The runoff turbulence enhanced with increasing slope lengths and rainfall intensities. The flow pattern were sluggish flow and laminar flow under2～10m slope length and30-150mm/h rainfall intensity on three land use types. Results from bare slope indicated that there was a positive correlation between runoff depth, Reynolds number, Froude number, runoff shear stress, runoff velocity and sediment yield at0.01level. The relativity between Reynolds number and sediment yield was the highest, which explained that the flow pattern had significant influence on shallow flow erosion force.3. The corresponding critical rainfall intensities on three land use types in red soil regionThe rainfall intensity and slope length were chosen as variables, we analyzed the slope length effect on runoff and sediment yield on three land use types, results showed that the bare slope with exposed soil surface and sloping farmland with artificial disturbance were more sensitive to rainfall intensity than economic forest slope,60mm/h rainfall intensity was the critical value that resulted in sudden increase of erosion for sloping farmland and bare slope, as a consequence, the soil and water conservation measures should be strengthened when rainfall intensity greater than60mm/h. For economic forest slope, there was a significant soil loss increase only when rainfall intensity greater than90mm/h due to the canopy interception and soil surface cover under forests. Therefore, the90mm/h rainfall intensity was critical value and the slope drainage establishment should be strengthened and we should pay more attention on maintenance and update of soil surface vegetation under forests to decrease the direct splash erosion by rainfall drops and erosion by runoff.4. Critical slope lengths on three land use types for soil and water conservation engineering measures in red soil regionThe critical slope lengths on three land use types for soil and water conservation engineering measures were revealed based on the study of slope length effect on runoff and sediment yield process. Results showed that the runoff volume and sediment amount increased as a whole with increasing slope length on three land use types, however, the increase of runoff volume and sediment yield didn’t in terms of equidifferent or geometric proportion tendency with increasing slope length. The soil and water loss amounts were lower and loss velocity were slower when slope length increased to certain length. Therefore, we suggest that the economic forest could be planted with4-6m net interval and the level trenches and level steps be constructed to truncate the runoff path and decrease soil and water loss. For sloping farmland, the intercepting ditch on slope could be constructed with4m interval to decrease runoff scour power and strengthen the rainfall infiltration, at the same time, the runoff could be accumulated to irrigate farmland. On the other hand, the vegetable fencing could be planted to prevent the soil coursing and decrease of land productivity due to soil and water loss and it could be used as green manure to improve soil organic matter content and soil moisture condition and decrease the fertilizer used amount. For the bare slope, we could construct the engineering measures or plant vegetation with4～6m interval to decrease soil and water loss.5. Conversion coefficients calculation of soil erosion modulus and runoff modulus between indoor and field experiments Based on the experiment results from field bare slope, we matched indoor erosion process simulation experiment on bare slope with similar proportion as field. Results indicated that the runoff volume and sediment amount were not multiple relationships corresponding to indoor and field area, a significant difference value between them does exist. Therefore, soil erosion modulus and runoff modulus were chosen as parameters to calculate the conversion coefficients between field and indoor in red soil region and the values were0.86and0.63, respectively.The innovative points for this study are as follows:1. Obtained the conversion coefficients of runoff modulus and soil erosion modulus between indoor and field experiments and a conversion method was proposedThe model simulated experiment is an important method for soil erosion research and the difficulty is whether the results from indoor experiment could be used to field exactly. Thus, we calculated the ratio of runoff modulus and soil erosion modulus between field and indoor experiments with different weights and obtained the conversion coefficients between field and indoor experiment and a conversion method was proposed, which could be used to predict field runoff modulus and erosion modulus objectively.2. Proposed the critical slope length and critical rainfall intensity for soil and water conservation under different land use typesThe changes of runoff and sediment yield with slope length were studied on economic forest slope, sloping farmland and bare slope, which are the main land use types on the low hilly gentle slope in Zhejiang province. Based on the hydrodynamics principle, we researched the relationship between hydrodynamics characteristic values and runoff and sediment yield, the critical slope length for soil conservation measures construction and critical rainfall intensity for soil and water conservation detection were obtained under different land use types.3. Revealed the non-arithmetical changes of runoff volume and sediment yield with the same increment of slope lengthSlope length effect research was basis for design of engineering measures on slope. Hence, the runoff and sediment yield simulated experiments were carried out under different slope lengths, we analyzed the slope length effect on runoff and sediment yield and revealed the non-arithmetical changes disciplinarian of runoff volume and sediment yield with the same increment of slope length.